About 20% of diamonds exhibit fluorescence. Fluorescence is usually observed in darkened rooms under UV lighting. This gives the diamond the appearance of a white shirt in a black light. Blue is the most common fluorescent color, ranging from a faint to very strong glow. Blue fluorescence can be an advantage, particularly in lower colored diamonds, because blue is the complimentary color to yellow. When shopping for a colorless diamond, make sure the fluorescence is blue. Any other color will distort the stone’s naturally clear appearance and degrade the value. If you are unsure about a diamond’s fluorescence, ask to view a full grading report before purchasing.

The GIA grades diamond fluorescence by level of strength: None, Faint, Medium, Strong, or Very Strong. When fluorescence is faint, it’s listed without noting the color of fluorescence. When a diamond exibits higher levels of fluorescence, the color of the fluorescence is also noted, and it is usually blue in nature.

It should also be said that too much fluorescence can be a problem, as the diamond can appear cloudy and decrease in value.